1. Field of the Invention
This invention relates to an automatic transmission for use in a vehicle and, more particularly, to an automatic transmission having a gear train having two single pinion type planetary gear sets and one double pinion type planetary gear set.
2. Description of the Prior Art
As is well known, a planetary gear set has three components, i.e., a sun gear, a ring gear and a carrier for holding a pinion gear meshing with the sun gear and the ring gear, wherein one component is defined as the input member, another component is defined as the output member and the other remaining component is held stationary, so that the input rotational speed is either increased or reduced forward or reversely to generate the output. Thus, in general, a plurality of planetary gear sets are combined to constitute a gear train for automatic transmission up to now. In this case, the obtained change gear ratio is changed variously according to how to combine the planetary gear sets, a value of the gear ratio (ratio of the number of teeth of sun gear to that of ring gear) of the planetary gear set and further which of the single pinion type planetary gear set and double pinion type planetary gear set is used, while all of the combinations cannot be necessarily put into practical use. Therefore, the gear trains that are possible in practical use are limited according to various requirements of the mounting property on a vehicle, the possibility of manufacture, the gear change characteristics, the required power performance or the like. In other words, since a great number of gear trains are constituted according to various combinations of planetary gear sets and ways of setting the gear ratios, it is very difficult to create the gear train satisfying the various requirements of the automatic transmission for use in a vehicle.
Conventionally, a number of automatic transmissions devised under such background have been proposed, and three of these automatic transmissions, each of which uses three planetary gear sets, are disclosed in Japanese Patent Laid-open Nos. 60-88252, 51-48062 and 54-132058, for example.
The automatic transmission disclosed in the above-mentioned Japanese Patent Laid-open No. 60-88252 is so constituted that three single pinion type planetary gear sets are combined to constitute a gear train, whereby five forward speeds and one reverse are permitted to set. However, in this automatic transmission, if the gear ratios of the respective planetary gear sets are so set that the change gear ratios of the respective gear change stages are approximately relevant to geometric series to prevent a drive force of a vehicle from remarkable reduction before and after the gear change, the gear ratio of any planetary gear set needs to be set to a remarkably large value, resulting disadvantageously in the enlargement of an outer diameter of the pertinent planetary gear set. On the contrary, if the gear ratios of the respective planetary gear sets are set to such an extent that the respective planetary gear sets do not particularly increase in outer diameter, the change gear ratios of the respective gear change stages are not generally relevant to geometric series to thereby bring about the deterioration of the power performance of the vehicle.
Also, the automatic transmission described in the above-mentioned Japanese Patent Laid-open No. 51-48062 is so constituted that two single pinion type planetary gear sets and one double pinion type planetary gear set are combined to constitute a gear train, whereby five forward speeds and two reverses are permitted to set. In this automatic transmission, however, when the gear change is made between the first gear and the second gear and between the second gear and the third gear, it is necessary to change the engaged and released conditions of four engaging means consisting of two clutch means and two brake means, so that shocks are worsened or complicated gear change control is inevitably done.
Further, the automatic transmission described in the above-mentioned Japanese Patent Laid-open No. 54-132058 is so constituted that one Ravignawx type planetary gear set and one single pinion type planetary gear set or two single pinion type planetary gear sets and one double pinion type planetary gear set are combined to constitute a gear train, whereby five forward speeds and one reverse are permitted to set. In this automatic transmission, however, a large load torque is exerted to sun gears of the respective planetary gear sets in the first and second gears and also in the reverse to be unfavorable to the strength and the durability. Also, since the power circulation takes place in the third gear, this automatic transmission is not only disadvantageous to the strength and the durability in that respect, but also it degrades the transmitting efficiency of power.